The primary function of the medical stethoscope is to accurately transmit vital signs sounds from a human being or animal. To assess these sounds the medical practitioner auscultates his/her patient and listens to diagnose heart sounds, breath sounds, lung sounds and bowel sounds.
While auscultating the patient, the practitioner forms a mental image of the sounds based on the tone, pitch, amplitude, intensity and sound quality of the incoming sound signal. By comparing this image with what the practitioner regards as "normal" sounds, such information about the patient's heart condition, blood pressure, respiration, bowel sounds etc. can be used to assess the patient's physical condition.
If the sound signal provided by the stethoscope is lacking in frequency range, intensity, or clarity, the practitioner will be limited by insufficient diagnostic information to construct an accurate assessment of the patient's condition. Likewise, if the delivered signal through his/her stethoscope is attenuated or distorted, a correct diagnosis is very difficult. Thus, it is important that the sound transmission be accurate.
Traditional stethoscopes include a flat circular diaphragm for application to the surface of the body part. The diaphragm is connected to a housing to which a first end of a listening tube is attached. The second end of the listening tube is placed into the medical practitioner's ears, usually with ear pieces.
Problems with traditional stethoscopes include: (a) most existing stethoscopes do not faithfully reproduce all human organ sound functions, or vital sounds; (b) low frequency sounds (below 80 Hz) are not transduced or are difficult to recognize; (c) the tone quality of the signal produced is flat and lacks needed brilliance and character; (d) a usable signal cannot be produced if clothing separates the instrument from bare skin; and (e) signal strength is insufficient to monitor patients in ambulances, aircraft or boats, due to high level background noise.
As to problems (a) through (c), since 1940 and earlier, stethoscope designs have been inherently simplistic, and attenuate, or even block transmission of medium-frequency and low-frequency (20 to 60 Hz) sound waves emitted from the heart, lungs, muscles and lower bowels. These lower frequency sound waves are common to heart activity, both normal and pathological, and to certain lung sounds. Lung sounds include, for example, rhonchi, which are described as continuous, sonorous, low-pitched sounds, associated with acute or chronic bronchitis.
An improved stethoscope design is described in U.S. Pat. No. 4,270,627 to Hill which includes a probe pick-up diaphragm head, elongate resonating tubes, a perforated circular outer flange, and a tube for connecting the device to conventional ear pieces. The disclosed structure, such as that of FIG. 8 of the patent, provides greater-volume, low-frequency sounds, but does not deliver the higher frequency vital signs practitioners are accustomed to hearing from conventional stethoscopes. And further, the low frequencies originating within the heart are distorted and sound boomy by this design, and are believed to produce muting of important diagnostic high frequencies, such as systolic and diastolic murmurs and pericardial rub which occur in the frequency range of 120 through 660 Hz. Lung sounds and breath-sounds, as a class, have a proportionately smaller amount of low frequency components than heart sounds. Abnormal lung sounds termed "rales" and "ronchi" contain a variety of higher-pitch sounds. These sounds are also muted in the Hill design.
The inventors of the subject invention modified the design disclosed in Hill to incorporate a second transmission channel between the device and the ear pieces, shortened the resonating tube, and developed a "one-piece" hemispherical polycarbonate diaphragm in place of the probe pick-up diaphragm head. These changes produced a significant improvement over the Hill design, in terms of balancing the delivery of low to high frequencies emitted by the human heart. This modified design has been successfully marketed under the trademark CARDIOSONIC ACOUSTIC AMPLIFIER Model C-2000, by Biosources International, Inc., Napa, Calif..
A significant drawback of the Hill design is the cost to manufacture the labor intensive coiled tube "pick-up head." More specifically, these designs contemplate the step-drilling of a number of precision holes in a base; cutting and fitting of rigid tubes to engage the flexible transmission tubing (dual lumen); the cutting, pre-bending and wrapping the elongate resonant tube; soft-soldering the elongate resonant tube into the step-drilled holes in the base; the drilling, then plugging of passageway-connecting holes in the base; and constant quality control inspections to ensure open sound passageways. Further, in order to minimize the physical size of the finished stethoscope, the elongate resonant tubing must be precision wrapped just one way.
Subsequent to the Model C-2000, the inventors of the subject invention developed a more manufacturable stethoscope design designated the KLIPPERT RESONATOR, Model KR700. This design features a one-piece, hemispherical diaphragm, like the Model C-2000, but adds a hemispherical cavity in the housing of the stethoscope, complementary to the hemispherical diaphragm, so that the two hemispheres combine to form a spherical, resonant chamber. Also included is a horn-shaped collector, positioned at a focal point in the spherical, resonant chamber to collect sound from the chamber for routing to the transmission tubing. While the KR-700 design exhibits certain improved performance over the Hill device and C-2000 design, and most certainly over traditional devices, there is room for improving the fidelity of the sounds produced.
The problems of lack of amplification, limited frequency response range, poor tone quality, and painful ear seals of existing medical stethoscopes also apply to stethoscopes employed by mechanics and engineers. These stethoscopes are used in assessing machinery operation, particularly for troubleshooting and repairing potential equipment problems.